TransportSceneManager: Prevent freeze when inserted from menu

[chapulina]

🦟 Bug fix

• Part of Cleanup GUI plugin menu gz-sim#1254

Summary

Currently, the TransportSceneManager freezes the window when inserted from the GUI menu, because it's waiting for an empty transport service for 30 seconds. With this PR:

• The service and other topics have default values, so the scene manager could potentially work when inserted
• The service and topics are displayed to the user, so they have an idea of what the plugin is trying to do
• The service request happens in a separate thread, so the rendering thread isn't blocked for 30 seconds
• If the user provides invalid topics, don't even subscribe to render events

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[ahcorde]

LGTM, just a small comment about the how to display the topic names

[codecov]

Codecov Report

Merging #365 (7cdf467) into ign-gui6 (ce4c368) will increase coverage by 0.09%.
The diff coverage is 75.75%.

@@             Coverage Diff              @@
##           ign-gui6     #365      +/-   ##
============================================
+ Coverage     65.39%   65.48%   +0.09%     
============================================
  Files            39       39              
  Lines          5123     5148      +25     
============================================
+ Hits           3350     3371      +21     
- Misses         1773     1777       +4     

Impacted Files	Coverage Δ
...s/transport_scene_manager/TransportSceneManager.cc	53.97% <75.75%> (+1.68%)	⬆️
src/plugins/scene3d/Scene3D.cc	49.87% <0.00%> (+0.25%)	⬆️

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